A conundrum develops with respect to the use of zinc electrodes in high rate, high performance batteries. On the one hand, the use of zinc electrodes provides low cost negative electrodes. On the other hand, when such electrodes are pasted, as is usually the case, they are very difficult and very expensive to make, especially when steps are taken to avoid shape change as will inevitably occur while the zinc electrode is cycled during the life of the rechargeable battery in which it is placed.
The present inventor has unexpectedly discovered that mass production of environmentally benign zinc electrodes can be provided for, by adapting certain aqueous pasting techniques, and by the provision of a stable zinc-based slurry or paste which particularly has an additional oxide of calcium, magnesium, or barium included therein for purposes of stability.
It is well known that rechargeable batteries having zinc electrodes have the advantage of using a low cost negative electrode. Of course, they also provide high voltage, and light weight.
On the other hand, the zinc electrode in alkaline batteries will exhibit some solubility in most alkaline electrolytes, resulting in the loss of integrity of the zinc electrode as the battery is cycled. This phenomenon of zinc electrode disfigurement is well known, and is referred to in the industry as shape change.
Many approaches have been taken to control shape change. The most successful attempts have limited the solubility of the zinc electrode, so as to thereby reduce its ability to migrate.
For example, EISENBERG U.S. Pat. No. 4,224,391, issued Sep. 23, 1980, and U.S. Pat. No. 5,215,836 issued Jun. 1, 1993, each provides electrolyte formulations that employ mixtures of potassium hydroxide and boric acid, phosphoric acid, or arsenic acid. The purpose, in all events, is to limit the solubility of the zinc electrode.
Another approach is that taken by ADLER et al in U.S. Pat. No. 5,302,475, issued Apr. 12, 1994. In that patent, the inventors teach an electrolyte which contains potassium hydroxide and a combination of potassium fluoride and potassium carbonate salts. The zinc electrode is reported to exhibit low shape change.
Yet another approach which is known to the inventor, is the addition of material such as calcium oxide to the electrode itself. This approach will modify the chemical composition of the discharge product to effectively lower the concentration of the zinc ion in the solution. However, in order to do so a method must be devised whereby a calcium zincate water based paste or slurry is manufactured, for pasting into the conductive substrate of a zinc electrode.
Moreover, the method by which the zinc electrode is manufactured may have significant effect upon cycle performance of the battery. A number of preparation processes have been used to further optimise the performance of zinc electrodes. They include powder pressing as taught by HEIN et al U.S. Pat. No. 3,645,793, issued Feb. 29, 1972. In that patent, the inventors teach pressing metallic powder into highly porous metallic battery electrodes structures, and the powders are first cleaned by the action of mild acids prior to and during the pressing operation.
Another approach is that taken by CHARKEY et al in U.S. Pat. No. 5,863,676, issued Jan. 26, 1999. Here, a non-aqueous sheet formation is employed, where a calcium zincate constituent is formed external of the battery, and then placed in the battery in layers.
However, any process which employs a pasting operation is difficult to control as far as managing the “set” time during pasting is concerned. This is reported in a paper entitled Pasted-Rolled Zinc Electrodes Containing Calcium Hydroxide for Use in Zn/NiOOH Cells by Gagnon et al, published in the Journal of the Electrochemical Society, September 1987, at pages 2091 to 2096. It is reported at page 2092 that the electrode fabrication was carried out in such a manner that the same amount of paste is added to both sides of a grid, by sandwiching the paste between backing papers and passing it through rollers to make a thin film. Thereafter, the current collector was placed between 2 films, and the composite was pressed to make a zinc electrode. This provided a longer set-time in which to paste the electrodes, and adhered the paste to the current collector grid in satisfactory manner. The thickness of the electrode, and therefore the zinc loading density, was controlled by the gap between the rollers.
Regrettably, none of the above mentioned processes can be readily transferred to mass production techniques, for high rate production of zinc electrodes for use in high performance batteries. Therefore, it has not been possible to provide a commercial product at a reasonable price, even though the chemical problems of the shape change of the zinc electrode have been effectively resolved.
The present invention provides a method for mass production of any environmentally benign zinc electrode, where the electrode will demonstrate excellent mechanical characteristics, good rate capability, and high cycle life, without significant shape change.
Indeed, the methods of the present invention may be readily and easily adaptable to all rechargeable zinc battery production, so that in time low cost zinc-based batteries may be manufactured in substantial quantities, and thereby replace toxic cadmium-based batteries. Such cadmium-based batteries enjoy a strong market position, even now, due to their relatively low cost, notwithstanding their toxicity and notwithstanding memory effect, and the like.
It is very difficult to uniformly coat an aqueous paste of zinc oxide onto a metallic current collector. When calcium oxide or calcium hydroxide is added to the mix, the problem of uniformly coating an aqueous paste zinc oxide onto a metallic current collector is even more difficult.
These problems come as a consequence of the rapid solidification which may prevent further work with the mix, or the final pasted plate may be structurally unsound in such a manner that further handling is impossible.
These problems, of course, have lead to non-standard manufacturing techniques in the manufacturer of zinc electrodes, thereby adding to the costs of the manufacture of the zinc electrodes. However, if a successful adoption of aqueous based manufacturing techniques such as those which are applied to cadmium electrodes could be developed for zinc-based electrodes, then a reliance on toxic cadmium-based batteries would be overcome.
The present invention provides such a solution. In keeping with the present invention, it is possible to manufacture a continuous coated electrode without setting problems, and to provide a mechanically sound electrode. Briefly, the present invention provides for a calcium zincate or other precursor constituent for an aqueous solution, based primarily on mixtures of zinc oxide with calcium oxide, magnesium oxide, or barium oxide, together with further optional metal oxides, and appropriate pasting constituents as a cellulose compound, a dispersant, and a binder.